The big rally follows the revelation in an SEC filing by Warren Buffett's holding company that it held 13,981,800 shares, a 6.4% stake, of the nation's biggest used-car retailer as of September 30, the end of Berkshire's third quarter. It's a new stake for Buffett and Berkshire. (Carmax plans to open a new location in Omaha about a month from now.)

The filing also shows that Berkshire continued to cut its stakes in Norfolk Southern and Union Pacific, even as it added to its Burlington Northern stake.

Berkshire also added to its holdings of U.S. Bancorp and Wells Fargo .